Electrode structure, and semiconductor light-emitting device having the same

ABSTRACT

A semiconductor light emitting device including: a transparent substrate; an electron injection layer which is formed on the transparent substrate; an active layer which is formed on a first region of the electron injection layer; a hole injection layer which is formed on the active layer; a first electrode structure which is formed on the hole injection layer and concurrently provides a high reflectivity and a low contact resistance; a second electrode structure which is formed on a second region of the electron injection layer; and a circuit substrate which is electrically connected with the first and second electrode structures, the first electrode structure includes: a contact metal structure which has any one selected from the group consisting of nickel, palladium, platinum and ITO (Indium Tin Oxide) that have low contact resistance; and a reflective layer which has aluminum or silver.

BACKGROUND OF THE INVENTION

This application claims the priority of Korean Patent Application No.2003-75220, filed on Oct. 27, 2003, in the Korean Intellectual PropertyOffice, the disclosure of which is incorporated herein in its entiretyby reference.

1. Field of the Invention

The present invention relates to a semiconductor light-emitting deviceusing a nitride semiconductor or a like material, and more particularly,to a high reflective electrode structure concurrently satisfying a lowcontact resistance and a high reflectivity, and a flip-chip lightemitting device having the same.

2. Description of the Related Art

A nitride-based compound semiconductor, which is generally used for avisible light emitting device, is being currently advanced to anultraviolet light region for a white light emitting device going througha visible light region of blue and green. The nitride-based compoundsemiconductor is mainly classified into a structure of extracting anupward light from light emitting from the active layer, and a structureof extracting a downward light passing through a transparent substratesuch as a sapphire substrate.

In a flip-chip light emitting device having the structure of extractinglight through the transparent substrate, reflectivity at an interface ofa P-type electrode is of importance to again reflect the upward light todirect downward.

In the meantime, it is advantageous that a light emitting device has alow operation voltage. At present, the most general method for loweringan operation voltage of the light emitting device decreases resistanceof a material layer formed between an electrode layer and an activelayer. Especially, since a hole injection layer and a P-type electrodeare Ohmic contacted each other in the flip-chip light emitting device,it is very desirable that the hole injection layer and the P-typeelectrode have low Ohmic contact resistance formed therebetween so as toreduce the operation voltage.

FIG. 1 is a schematic sectional view illustrating a conventional nitridesemiconductor light-emitting device.

As illustrated in FIG. 1, the conventional flip-chip nitridesemiconductor light emitting device 10 includes a sapphire substrate 11;an N-type GaN layer 12 sequentially formed on the sapphire substrate 11;an active layer 16 formed of InGaN; a P-type GaN layer 18; a nickellayer 20; a P-type reflective electrode 22; and an N-type electrode 14formed on one side surface of the N-type GaN layer 12. The lightemitting device 10 has a dual hetero structure where the N-type GaNlayer 12 functions as a cladding layer for a first conductive type, andthe P-type GaN layer 18 functions as a cladding layer for a secondconductive type.

Further, the nickel layer 20 is formed on the P-type GaN layer 18 tohave a thickness of below about 10 nm, and functions as a contact metallayer for forming the Ohmic contact. Since the P-type reflectiveelectrode 22 is formed of aluminum (Al) or silver (Ag), lighttransmitting the nickel layer 20 that is the contact metal is reflectedat an interface between the P-type reflective electrode 22 and thenickel layer 20.

The conventional light emitting device 10 can directly extract lightfrom the P-type reflective electrode that is formed of material such asaluminum (Al) or silver (Ag) with a high reflectivity, and can obtain ahigh efficiency of light extraction. However, the conventional lightemitting device has a disadvantage in which contact resistance isincreased when the P-type reflective electrode 22 with the highreflectivity is directly employed. Accordingly, the nickel layer 20 isformed as the contact metal for forming the Ohmic contact, therebyreducing the contact resistance.

However, in the flip-chip nitride semiconductor light emitting device 10having the nickel layer 20 as the contact metal, since light emittingfrom the active layer 16 formed of InGaN passes through the nickel layer20, and then is reflected at the interface of the nickel layer 20 andthe P-type reflective electrode 22, and then again passes through thenickel layer 20 and the sapphire substrate 11 for emission, a largeamount of light is absorbed by the nickel layer 20. Therefore, theconventional flip-chip nitride semiconductor light emitting device 10has a drawback in that it is very difficult to increase thereflectivity.

In other words, since the nickel layer 20, which is the contact metal,is used to be in reliable contact with the P-type GaN layer 18, thethicker nickel layer 20 can provide a better contact with the P-type GaNlayer 18. However, if the nickel layer 20 has a thickness of above 10nm, it is difficult to have enough reflectivity.

Accordingly, the semiconductor light emitting device is required to havea reflection structure for maintaining the high reflectivity whilemaintaining the low contact resistance.

SUMMARY OF THE INVENTION

The present invention provides a semiconductor light-emitting devicehaving a P-type electrode structure concurrently satisfying a lowcontact resistance and a high reflectivity.

Further, the present invention provides an electrode structureconcurrently satisfying a low contact resistance and a high reflectivityin a semiconductor light-emitting device.

According to an aspect of the present invention, there is provided asemiconductor light emitting device including: a transparent substrate;an electron injection layer which is formed on the transparentsubstrate; an active layer which is formed on a first region of theelectron injection layer; a hole injection layer which is formed on theactive layer; a first electrode structure which is formed on the holeinjection layer and concurrently provides a high reflectivity and a lowcontact resistance; a second electrode structure which is formed on asecond region of the electron injection layer; and a circuit substratewhich is electrically connected with the first and second electrodestructures, wherein the first electrode structure includes: a contactmetal structure which has any one selected from the group consisting ofnickel, palladium, platinum and ITO (Indium Tin Oxide) that have lowcontact resistance; and a reflective layer which has aluminum or silver.

According to another aspect of the present invention, there is providedan electrode structure including: a transparent substrate; an electroninjection layer which is formed on the transparent substrate; a contactmetal structure which has any one selected from the group consisting ofnickel, palladium, platinum and ITO that have low contact resistance tobe used in a semiconductor light emitting device having an active layerand a hole injection layer; and a reflective layer having the highreflectivity such as aluminum or silver.

Here, the contact metal structure may be island-type or mesh-type.

An area ratio of the contact metal structure to the reflective layer maybe from 1% to 90%, and the thickness of the contact metal structure isless than 200 nm.

Further, the reflective layer may be formed of aluminum (Al) or silver(Ag) that has the high reflectivity.

Furthermore, the transparent substrate may be formed of sapphire orsilicon carbide.

Also, the electron injection layer may be formed of N-type GaN, theactive layer may be formed of InGaN, and the hole injection layer may beformed of P-type GaN.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent by describing in detail exemplary embodimentsthereof with reference to the attached drawings in which:

FIG. 1 is a schematic sectional view illustrating a conventional nitridesemiconductor light-emitting device;

FIG. 2 is a sectional view illustrating a semiconductor light-emittingdevice according to a preferred embodiment of the present invention;

FIGS. 3A and 3B are sectional views illustrating an electrode structureused in a semiconductor light-emitting device of FIG. 2 according to apreferred embodiment of the present invention;

FIGS. 4A through 4F are plane views illustrating electrode structuresdepending on varied area ratios of a palladium layer to a silver layeraccording to the present invention; and

FIGS. 5A and 5B are graphs illustrating the correlation of lightemission and respective meshed regions shown in FIGS. 4A through 4F, andthe correlation of operation voltage and the respective meshed regionsin a semiconductor light-emitting device.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described more fully with reference tothe accompanying drawings, in which exemplary embodiments of theinvention are shown. The invention may, however, be embodied in manydifferent forms and should not be construed as being limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the concept of the invention to those skilled in the art. In thedrawings, the thicknesses of layers and regions are exaggerated forclarity. It will also be understood that when a layer is referred to asbeing “on” another layer or substrate, it can be directly on the otherlayer or substrate, or intervening layers may also be present.

FIG. 2 is a sectional view illustrating a semiconductor light-emittingdevice according to a preferred embodiment of the present invention.

As shown in FIG. 2, the semiconductor light-emitting device 100 includesa transparent substrate 102 formed of transparent material such assapphire (Al₂O₃) or silicon carbide (SiC); an electron injection layer104 formed of an N-type GaN on the transparent substrate 102; an activelayer 106 formed of InGaN; and a hole injection layer 108 formed of aP-type GaN. The electron injection layer 104 includes a first portionand a second portion, and step-shaped with the first portion thinnerthan the second portion. The active region 106 and the hole injectionlayer 108 are formed on the second portion.

Further, the semiconductor light-emitting device 100 further includes aP-type electrode structure 110 also functioning as a reflective layerformed on the hole injection layer 108 according to a preferredembodiment of the present invention, the P-type electrode structure 110may include a contact metal structure functioning as a contact metalforming Ohmic contact to reduce contact resistance; and a reflectivelayer formed of metal such as silver (Ag) or aluminum (Al) with the highreflectivity.

FIGS. 3A and 3B are sectional views illustrating an electrode structureused in the semiconductor light-emitting device of FIG. 2 according to apreferred embodiment of the present invention.

Referring first to FIG. 3A, the P-type electrode structure 110 accordingto a first embodiment of the present invention is formed on the holeinjection layer 108, and includes a contact metal structure 110A thatfunctions as the contact metal forming the Ohmic contact to reduce thecontact resistance; and a reflective layer 110B. Additionally, thecontact metal structure 110A may be formed of any one selected from thegroup consisting of nickel (Ni), palladium (Pd), platinum (Pt) or indiumtin oxide (ITO) that have a low contact resistance. The reflective layer110B may be formed of metal having the high reflectivity such asaluminum (Al) or silver (Ag).

Further, the P-type electrode structure 110 according to a preferredembodiment of the present invention performs a function of uniformlydistributing current, which is applied from a circuit board assembledlater on, in the hole injection layer 108, as well as a function ofcontact.

In the meantime, in the active layer 106, electrons injected from theelectron injection layer 104 are combined with holes injected from thehole injection layer 108. The combined electrons and holes fall to a lowenergy band to cause light emission. At this time, the emitting light isreflected at an interface between the reflective layer 110B and thecontact metal structure 110A of the P-type electrode structure 110 andat an interface between the reflective layer 110B and the hole injectionlayer 108. The reflected light sequentially goes through the holeinjection layer 108, the active layer 106, the electron injection layer104 and the transparent substrate 102 while emitting in the direction ofan arrow of FIG. 2.

The contact metal structure 110A is island-shaped, and the reflectivelayer 110B covers the resultants including the hole injection layer 108and the contact metal structure 110A. Though the contact metal structure110A is rectangular island-shaped, but can have other shapes such as asemispherical shape or a regular-tetrahedron within a scope or spirit ofthe present invention.

Referring to FIG. 3B, a P-type electrode structure 210 according to asecond embodiment of the present invention is formed on the holeinjection layer 208, and includes a contact metal structure 210A thatfunction as the contact metal forming the Ohmic contact to reduce thecontact resistance; and a reflective layer 210B. The reflective layer210B may be formed of metal having the high reflectivity such asaluminum (Al) or silver (Ag).

The contact metal structure 210A is mesh-shaped, and the reflectivelayer 210B covers the resultants including the hole injection layer 208and the contact metal structure 210A. Though the mesh-shaped contactmetal structure 210A has a square bar shape, but can have other shapessuch as a cylindrical shape or a rectangular shape within a scope orspirit of the present invention.

Referring again to FIG. 2, an N-type electrode 112 is formed on thethinner first portion of the electron injection layer 104. The N-typeelectrode 112 can be also formed to have an electrode structure such asTi/Al/Pt/Au in which metals are deposited. As described above, aftersemiconductor light-emitting parts are mounted on the transparentsubstrate 102, the resultant transparent substrate 102 is aligned on asub-mount 118 having an Au layer 116 and a solder ball 114 formed on theAu layer 116. The Au layer 116 is wire-shaped such as a lead frame.

Next, flip-chip bonding is performed to assemble the sub-mount 118 withthe transparent substrate 102 mounting the semiconductor light-emittingparts thereon, so that the semiconductor light-emitting device 100 iscompleted. Though not illustrated in detail in the drawings, but aprocess of forming a bonding metal for bonding the P-type electrodestructure 110 and the N-type electrode 112 with the sub-mount 118 can beadditionally performed.

FIGS. 4A through 4F are plane views illustrating the electrodestructures depending on varied area ratios of the palladium (Pd) layerto the silver (Ag) layer according to the present invention.

FIGS. 4A through 4F illustrate a contact metal layer formed of palladium(Pd) that is formed on the hole injection layer to have a thickness ofabout 3 nm. This represents experimental results of the electrodestructures where the area ratios of the Pd layer to the Ag layer arevaried going from FIG. 4A to FIG. 4F so as to describe the effect of thepresent invention.

Describing in detail, in FIG. 4A, all portions denoted by 402 are formedas the Pd layer that is a standard type with a thickness of about 25-35Å. In FIGS. 4B through 4E, regions (mesh_1 to mesh_4) 402 respectivelycorrespond to the Pd layer. Each area ratio of the Pd layer to the Aglayer of the regions is 1.25, 0.78, 0.56 and 0.44, respectively. Here, areference numeral 400 denotes the reflective layer formed of Ag, and areference numeral 402 denotes the contact metal layer formed of Pd. InFIG. 4F, only the Ag layer is formed without the Pd layer.

FIGS. 5A and 5B are graphs illustrating the correlation of lightemission with respective meshed regions shown in FIGS. 4A through 4F andthe correlation of operation voltage with respective meshed regions in asemiconductor light-emitting device.

FIG. 5A illustrates the correlation of the light emission of the lightemitting device in the standard electrode structure, the mesh_1 tomesh_4 electrode structure and the electrode structure with only the Aglayer that are shown in FIGS. 4A through 4F.

As shown in the drawing, it can be appreciated that luminance is lowestin the standard electrode structure with the Pd layer being entirelyformed as the contact metal layer and then, the Al layer being entirelyformed on the Pd layer. As described above, this phenomenon occursbecause light emitting from the active layer is reflected at aninterface between the Pd layer and the P-type electrode structure suchthat the reflected light emits toward the transparent substrate, therebybeing much absorbed into the Pd layer.

According to a preferred embodiment of the present invention, when theelectrode structure employs the combination of the Pd layer and the Aglayer, luminance is improved by above 10%. Further, as the area ratio ofthe Ag layer to the Pd layer is decreased, the luminance is graduallyincreased. In the meantime, the standard electrode structure has thelowest luminance, and the electrode structure with only the Ag layer hasthe highest luminance.

FIG. 5B illustrates the correlation of the operation voltage of thelight emitting device in the standard electrode structure, the mesh_1 tomesh_4 electrode structure and the electrode structure with only the Aglayer that are respectively shown in FIGS. 4A through 4F.

As shown in FIG. 5A, the electrode structure having only the Ag layerwithout the Pd layer has the highest luminance in the light emittingdevice, but has the operation voltage exceeding 4.99V as shown in FIG.5B. Accordingly, the electrode structure has the operation voltagegreatly exceeding 3.80V that can be applied to an actual device. This isbecause the contact resistance is increased in the electrode structurewith only the Ag layer.

However, when the electrode structure employs the combination of the Pdlayer and the Ag layer according to a preferred embodiment of thepresent invention, the electrode structure has the operation voltagethat is not so large in comparison with the conventional electrodestructure while providing the high reflectivity such that the lightemission of the light emitting device can be maintained. Further, thepresent invention controls the area of the Pd layer to control theoperation voltage and the reflectivity of the P-type electrodestructure, thereby optimizing light efficiency of the light emittingdevice.

As described above, the present invention has an effect in that lightabsorption made by the contact metal layer can be reduced while lightefficiency of the semiconductor light emitting device can be improved,by controlling the area of the contact metal layer that is in contactwith the hole injection layer formed of the P-type semiconductor.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

1. A semiconductor light emitting device comprising: a transparentsubstrate; an electron injection layer formed on the transparentsubstrate; an active layer formed on a first region of the electroninjection layer; a hole injection layer formed on the active layer; afirst electrode structure which is formed on the hole injection layerand concurrently providing a high reflectivity and a low contactresistance; a second electrode structure formed on a second region ofthe electron injection layer; and a circuit substrate electricallyconnected with the first and second electrode structures.
 2. The lightemitting device of claim 1, wherein the first electrode structurecomprises: a contact metal structure which has any one selected from thegroup consisting of nickel, palladium, platinum and ITO (Indium TinOxide) that have low contact resistance; and a reflective layer formedon the contact metal structure.
 3. The light emitting device of claim 2,wherein the contact metal structure is island-shaped.
 4. The lightemitting device of claim 2, wherein the contact metal structure ismesh-shaped.
 5. The light emitting device of claim 3, wherein an arearatio of the contact metal structure to the reflective layer is from 1%to 90%.
 6. The light emitting device of claim 5, wherein the thicknessof the contact metal structure is less than 200 nm.
 7. The lightemitting device of claim 4, wherein an area ratio of the contact metalstructure to the reflective layer is from 1% to 90%.
 8. The lightemitting device of claim 7, wherein the thickness of the contact metalstructure is less than 200 nm.
 9. The light emitting device of claim 2,wherein the reflective layer is formed of aluminum (Al) or silver (Ag)that has a high reflectivity.
 10. The light emitting device of claim 1,wherein the transparent substrate is formed of sapphire.
 11. The lightemitting device of claim 1, wherein the hole injection layer is formedof P-type GaN.
 12. An electrode structure used in a semiconductor lightemitting device having an active layer and a hole injection layer formedon one surface of the active layer, the structure comprising: a contactmetal structure which is formed on one surface of the hole injectionlayer to face with the active layer, and has any one selected from thegroup consisting of nickel, palladium, platinum and ITO that have lowcontact resistance; and a reflective layer formed on the contact metalstructure.
 13. The electrode structure of claim 12, wherein the contactmetal structure is island-shaped.
 14. The electrode structure of claim12, wherein the contact metal structure is mesh-shaped.
 15. Theelectrode structure of claim 13, wherein an area ratio of the contactmetal structure to the reflective layer is from 1% to 90%.
 16. Theelectrode structure of claim 15, wherein the thickness of the contactmetal structure is less than 200 nm.
 17. The electrode structure ofclaim 14, wherein an area ratio of the contact metal structure to thereflective layer is from 1% to 90%.
 18. The electrode structure of claim17, wherein the thickness of the contact metal structure is less than200 nm.
 19. The electrode structure of claim 12, wherein the reflectivelayer is formed of aluminum (Al) or silver (Ag) that has a highreflectivity.
 20. The electrode structure of claim 12, wherein the holeinjection layer is formed of P-type GaN